1. Technical Field
The present invention relates to the removal of chloride ions from reinforced concrete structures, such as bridge decks and substructures, and more specifically, to a novel apparatus for such removal.
2. Description of the Prior Art
It is generally acknowledged that a major cause of the deterioration of reinforced concrete structures, such as bridge surfaces and substructures, is the corrosion of reinforcing steel due to the ingress of chloride ions from de-icing salts or sea water. Chloride causes active corrosion by destroying the natural passivity of steel in the alkaline environment of concrete. Prevention of corrosion of the steel components by electrochemical means can be accomplished by either applying cathodic protection or by removal of sufficient chloride to allow the steel to repassivate and then prevent the future ingress of chloride ion.
Prior publication entitled "Electrochemical Removal of Chlorides From Concrete Bridge Decks", Materials Performance, November, 1976, pages 21-26, by J. E. Slater et al. discloses the electrochemical removal of chloride ions by applying an anode and electrolyte to the structure surface and passing current between the anode and the reinforcing steel as a cathode. Since anions migrate toward the anode, it is possible to migrate chloride ions away from the steel and out of the concrete structure. The speed at which this process is accomplished is dependent largely on the magnitude of the applied current. In the Slater et al. procedure, a liquid electrolyte of calcium hydroxide dissolved in water was used, formed as ponds on the surface of the bridge deck. The electrolyte contained an ion exchange resin slurried into the electrolyte. The anode was platinized titanium. Direct current power leads were attached to the metal anode and to the steel reinforcing bars in the bridge deck. The chloride removal was undertaken using a direct current varying between about 1 and 2.5 amps per square foot with a voltage from 100 to 120 volts. The article reported that up to 90% of the chloride ions in the bridge deck could be removed in a 24-hour period.
Prior U.S. Pat. No. 4,832,803 teaches extracting chlorides from a reinforced concrete structure by attaching a thin net electrode to a surface of the concrete structure, and then applying a thin viscous chloride absorbing electrolyte material such as gunite to the surface of the concrete structure to cover the net electrode. A direct current between the reinforcing members and the net electrode causes chloride ions to migrate to and be absorbed in the electrolyte layer. One suggested electrode net was graphite fibers having high conductivity. An object in prior U.S. Pat. No. 4,832,803 was to provide a method which could be used not only on horizontal surfaces but also surfaces which were other than horizontal surfaces.
A Federal Highway Administration report number FHWA-RD-76-60 entitled "Neutralization of Chloride in Concrete", D. R. Lankard and others, 1975, discloses on page 96, using a self-supporting sponge containing an electrolyte. A metal anode can be permanently attached to the sponge surface. Alternatively, a graphite cloth anode can be made an integral part of the sponge, for instance as a filler in a sponge sandwich. The electrolyte in the sponge can be replenished as necessary, for instance by spraying the upper surface of the sponge/anode combination. An ion exchange resin can be spread on the deck surface before placement of the sponge, and can be successfully held in position by the sponge, particularly if the bottom surface of the sponge has a ribbed or waffled texture. It is mentioned in the report that the sponge concept is attractive because of its ease of placement, its portability, and its ease of reuse. From the context of the report, it appears that the authors contemplated treating only substantially horizontal surfaces, albeit surfaces that could have surface irregularities and gradients.